http://www.realmofthemadgod.com/you can play immediately, or create a quick account.
Basically run around and shot things:
some advice:
don't grind, just use the minimap to teleport into big groups which are often found hurtling across the map towards the centre.

Yeah, it is... very arcadey, very transient. The cowards until they're a little too slow on the uptake, and then they rage for a bit. We should try and get a train together sometime, those are massively fun.